Kites have been used for centuries for a variety of purposes associated with art, sport, and military reconnaissance. As used herein, the term “kite” broadly includes any object that is tethered, and remains in the air under power of the wind. This includes, for example, common play kites, a tethered blimp (also known as an aerostat), and a tethered toy or other airplane, or rotorcraft.
It has recently been proposed to use kites to extract mechanical or electrical power from high altitude winds. The most promising of these ideas involve kites maneuvering at high speed in a flight pattern approximately cross-ways to the wind. Figure-eight (FIG. 1) or circular flight patterns have been proposed, but many variations are possible. It can easily be shown that a kite maneuvering in such a pattern can attain crosswind speeds far greater than the speed of the wind itself, with a fundamental limit of L/D, where L/D is the ratio of aerodynamic lift to aerodynamic drag. For a high performance kite, L/D can be up to 15 or more.
Several methods of generating power using kites have been described in the literature (see for example, Loyd, M. L., Crosswind Kite Power, Journal of Energy, vol. 4, May-June 1980, p. 106-111), including most notably lift power extraction, and drag power extraction. Lift power extraction uses kite tether tension to unwind a ground-mounted spool which turns a generator or other power conversion device. Drag power extraction uses the kite's crosswind lift to propel a wind turbine or other power extraction device through the air at high speed. By neglecting the weight of the kite and the weight and drag of the tether, it can easily be shown that both lift and drag power extraction methods offer the same power generation potential:
  P  =            4      27        ⁢                  C        L        3                    C        D        2              ⁢          S      ref        ⁢          P      W      
where                CL and CD are the kite's lift and drag coefficients,        Sref is the reference area of the kite (usually the kite's projected planform area), and        
      P    W    =            1      2        ⁢    ρ    ⁢                  ⁢          V      W      3      is the kinetic power density in the wind, where                ρ is the mass density of the air, and        VW, is the wind speed.        
The value SrefPw is proportional to the amount of power a conventional wind turbine of turbine-disk area Sref could extract from a terrestrial wind flow. The value of
      4    27    ⁢            C      L      3              C      D      2      can be as high as 30 to 50 for high performance kites traveling at high speed. This means that a properly engineered power kite might extract 30 to 50 times as much power as a conventional wind turbine of equivalent size.
All kite power extraction methods suffer severe performance penalties associated with the weight and drag of the tether. The degree of performance penalty depends principally on the value of Ltether/√{square root over (Sref)}, where Ltether is the length of the tether from its anchor point to the kite attachment. Much of the tether drag penalty stems from the high drag coefficient of cylindrical cross section tethers and from the high flight speeds needed for efficient power extraction, so there is a benefit in keeping the portion of the tether which travels at high speed as short as possible. It is well known that winds above approximately 10,000 ft blow with greater intensity and are more reliable than wind near the ground. This motivates power kite designers to consider tethers up to Ltether=20,000 ft or longer. Maintaining a Ltether/√{square root over (Sref)} value below 100 to avoid excessive tether drag losses, the smallest feasible kite size is Sref=50,000 ft2.
A variation on the drag power extraction method has been proposed, which uses kites with rotating rotors (see for example Roberts, B. W., et. al., Harnessing High Altitude Wind Power, IEEE Transaction on Energy Conversion, vol. 22, issue 1, March 2007, p, 136-144). In this method, a kite which includes rotating rotors generates power onboard the aircraft, and some of this power is transmitted to the ground using conductors integrated with the kite tether. The electrical power is generated by using torque from the rotor shaft to turn a generator on the aircraft. The method of Roberts, et. al., is similar to the drag power method proposed by Loyd because the rotor blades are being propelled through the air in a cross-wind fashion, and it is the cross-wind component of the blade-lift which is used to produce power.
A Y-harness tether configuration (FIG. 2) has been proposed to minimize tether drag without requiring excessively large kites. Such an arrangement is comprised of a long-static tether whose upper end is the anchor point for the shorter tethers of two or more separate kites. If the several kites can be controlled to fly in opposing patterns such that their common anchor point is subject to zero net force, then the anchor point will remain stationary. Since drag forces grow with the square of the air speed, the long lower part of the tether will not be subject to the high drag associated with the high-speed maneuvering flight, but the much lower drag associated with the wind speed.
In the lift power extraction method, the tether spool unwinds during power generation and must be rewound back onto the spool while not generating. This method is incapable of sustaining continuous power generation because of the need to rewind the tether, but the power required to rewind the tether can be a small fraction of the power generated during tether unwind.
In the drag power extraction method, electrical power is generated onboard the kite and transmitted to the ground through conducting cables integrated with the tether. Though the drag power extraction method offers the possibility of continuous power generation, the weight, drag, cost, and risk of the electrical conductors and airborne generating equipment reduce the potential for economic utility.
Two methods have been proposed which answer the need for continuous power generation without the cost, weight, drag, and risk associated with airborne conductors and generators. These methods are the Ladder Mill, and the Kite Wind Generator, each of which involves a series of kites tethered to ground-based generating equipment. Both methods introduce considerable logistical complexity in the control and management of the kites and their tethers. Each of these methods also pays a large performance penalty because neither method is conducive to the high-speed crosswind maneuver patterns that allow the most efficient energy extraction.
The preceding discussion has focused on operation of kite systems once they are airborne. Many of the proposed systems present logistical challenges for launch and recovery of the kites, especially in cases where the kites are too large to be hand-launched by human operators.
Given the prior art, a need still exists for a method of extracting power from high altitude winds which allows continuous power generation without the cost, weight, drag, and risk of airborne conductors and generators, but which avoids the logistical complexity and performance penalties of the Ladder Mill and the Kite Wind Generator, minimizes the effects of tether drag, and provides a natural method for launching large-scale kites.